At OsmoDevCon2016 we discussed problems with our past contribution / patch submission process using mails on the mailing list as well as patchwork. The result is that we want to give Gerrit a try for some time and see if it helps us to have a better process

If you need to adjust and re-submit patches the easiest way is to create throw-away branch ("R D" in magit-gerrit in emacs for example), make you changes/amendments and than send patch(es) back to gerrit while removing temporary branch automatically with "git review -f".

There are different merge strategies that Gerrit performs to accept patches.Each project can be configured to a specific merge strategy, but unfortunately you can'tdecide on a strategy per patch submission.

It seems that the "Merge if Necessary" strategy is best supported, but it creates non-linearhistory with numerous merge commits that are usually not at all necessary.

Instead, the "Cherry Pick" strategy puts each patch onto current master's HEAD to createlinear history. However, this will cause merge failures as soon as one patch depends onanother submitted patch, as typical for a feature branch submission.

So we prefer the "Rebase if Necessary" strategy, which always tries to apply your patches tothe current master HEAD, in sequence with the previous patches on the same branch.However, some problems still remain, including some bugs in "Rebase if Necessary".

There are in general three problems with "Rebase if Necessary", with different solutions:

If your branch sits at master's HEAD, Gerrit may refuse to accept the submission, becauseit thinks that no new changes are submitted. This is a bug in Gerrit, which holger hasfixed manually in our Gerrit installation.

Say you have submitted a branch with 5 commits, and the third commit has gotten a -1 vote.You have fixed the problem in your local git and now would like to re-submit it. Of courseyou want to keep the Change-Ids identical so that Gerrit shows them as amends.

Gerrit will refuse to accept your patch submission if the first commit on the branch isstill identical.

The quickest work-around: slightly reword the first branch commit's commit log message.Yes, that's enough! All commits that are identical on the branch are now ignored,it's only the first branch commit that Gerrit will complain about.

The cause: Gerrit refuses to accept a commit with a Change-Id that it already knows andwhere the commit hash is identical.

Some details: you could modify all the Change-Ids, but now your branch submission wouldopen entirely new review entries and you would have to abandon your previous submission.Comments on the first submission are lost and you cannot diff between patch sets.Now, if you just cosmetically tweak the first commit's log message, the commit hashis changed. Since the following commits contain their predecessor's commit hash, nowall of the branch's commit hashes are modified, and gerrit happily accepts them as a new patch set. It will still pick up the Change-Ids (which you shouldn't edit) and notice if commits have remained identical (keeping the votes). But with the minorcommit log tweak, it will no longer thwart your re-submission with an error message.

Say you have an extensive feature in development, and you want to keep it on theupstream git repository to a) keep it safe and b) collaborate with other devs on it.So, of course, you have regularly pushed to refs/heads/yoyodyne/feature.

Since you have the gerrit commit hook installed, your feature branch already hasChange-Id tags in all commit log messages.

Now your feature is complete and you would like to submit it to master.Alas, Gerrit refuses to accept your patch submission for master, because itknows the Change-Ids are also on a different branch.

Gerrit by default enforces that a Change-Id must be unique across all branches,so that each submission for review is separate for each branch. Instead, wewant to handle Change-Ids per-branch, so that you can have the same changesubmitted to different branches, as separate patch submissions, without havingto cosmetically adjust the Change-Id.

Solution: set the option Create a new change for every commit not in the target branch to TRUE

By default, gerrit compares patches only by the files' paths. If two paths are the same,it immediately shows them as conflicts (path conflicts).

In software development, a conflict usually means an actual content conflict, so if theedits are in two entirely separate places in the file, we don't consider this a conflict.

By setting 'Allow content merges' to TRUE in the git project config, we tell Gerrit toperform text merges of the submitted patches and only complain about actual contentconflicts, in the usual software engineering sense.